CFD based wall condensation model for evaluating PCV conditions in Fukushima Daiichi Unit-1

Abstract Even after seven years since the Fukushima accident, conditions inside Fukushima Daiichi Unit-1 (1F1) remain mostly unknown, making decommissioning activities challenging. Furthermore, practical data from within the primary containment vessel (PCV) of 1F1, which may provide crucial information for decommissioning activities, have not been assessed sufficiently. In this study, an alternative approach for evaluating such practical data is investigated. In this approach, the condition of debris inside the PCV is estimated through computational fluid dynamics (CFD) simulations performed using STAR-CCM+. Heat transfer inside the PCV is modeled with the developed wall condensation. The wall condensation model is firstly validated through experiment results from COPAIN tests and applied to the PCV of 1F1. Possible locations of debris and decay power that dissipated and heated the environment inside the PCV are then estimated using the simulation results. Moreover, temperature asymmetry observed from practical data on safety valves is also addressed by conducting parameter analysis for different decay powers. The results suggest that 5% of decay heat (65 kW) dissipated to heat gas inside the PCV, and temperature asymmetry on safety valves could be explained by 2% of decay heat (20 kW) as the heat source. A significant amount of debris is estimated to exist inside the pedestal and safety relief valve locations, and some may have spread to the drywell floor through the doorway opening of the pedestal. The findings of the study contribute towards improved decommissioning of 1F1.